MS460 diagnosing fried piston/cylinder

[challenger]

I took apart the MS 460 I am working on for a friend but before tearing it down I sealed off the exhaust port and the intake. I applied pressure and vacuum through the fitting for the impulse tube and the crank case held pressure and vacuum. I sprayed soapy water all over the saw and didn't see leakage.
I have 2 places I may be able to place blame on why the cylinder & piston need replacing. The first is the impulse line to the carburetor. This saw recently had a new gas tank installed and I am suspicious that the impulse line was not installed properly. Could this allow the saw to run yet make it so lean it fried the piston?
The next area I think could be trouble is the muffler gasket between the 2 parts. The muffler was banged up (any saw not have a dented muffler?) and the gasket is broken where the two parts mate. I am wondering if this could allow rain water to enter the exhaust port if the saw was in the rain and in a position that could allow water to collect in it?
I had to order a complete seal kit in order to get the muffler gasket. Is it wise to reseal the saw even though it doesn't leak. I would not think so but thought I would ask some with more experience.
Thanks
Howard

 

[nautilus]

Doesn't seem like it's worth it to crack the case if you didn't find any leaks.
Is it possible that the carb was just adjusted too lean?
Does the saw have a new air filter?
You might be in luck with the cylinder, I just pulled the top end of my 046 that had a fried piston and the jug was perfect, no aluminum deposits or anything.

 

[7hpjim]

I would definitely do the crank seals so as to eliminate a possible cause of lean seize condition, is the piston scored only on exhaust if so lean seize and anything that can add too much air will cause it, including op error, is the carb set correctly, around 1 to 1 1/2 turn out from lightly seated,replace fuel and vent lines too just to make sure, loose manifold bolts may have caused this one and the op kept on running until failure, lucky the con rod didnt overheat
http://www.arboristsite.com/chainsaw/165265.htm

 

[7hpjim]

Doesn't seem like it's worth it to crack the case if you didn't find any leaks.
Is it possible that the carb was just adjusted too lean?
Does the saw have a new air filter?
You might be in luck with the cylinder, I just pulled the top end of my 046 that had a fried piston and the jug was perfect, no aluminum deposits or anything.

do you have to open case on a sthil to replace seals?

 

[mtngun]

do you have to open case on a sthil to replace seals?

No, but if the seals pass a vac/pressure test, I can't see any reason to mess with them.

Pics of the fried piston and jug would help.

Besides an air leak, a saw can also run lean due to a plugged fuel filter, a torn fuel line, a plugged carb screen, or a plugged tank vent.

Or it could have simply been straight gassed.

The muffler gasket is not normally very important.

 

[caleath]

I had a customer with a burned up one. The only thing I found was a fuel line that was collapsed. I would replace all lines and seals and intake boot.

 

[nautilus]

do you have to open case on a sthil to replace seals?

My apologies, I thought he was referring to replacing the crank gasket as well since he said he bought the whole set.

 

[Grande Dog]

Howdy,
The impulse line is used to drive the carb diaphragm pump and provide fuel to the carb during idle, low, and midrange rpm's. A saw can run at high speed with the the impulse line plugged because of the venturi in the carb. If there was an issue with the impulse line pervious to the failure, there would have been a noticeable operational problem.
Regards
Gregg

 

[THALL10326]

I took apart the MS 460 I am working on for a friend but before tearing it down I sealed off the exhaust port and the intake. I applied pressure and vacuum through the fitting for the impulse tube and the crank case held pressure and vacuum. I sprayed soapy water all over the saw and didn't see leakage.I have 2 places I may be able to place blame on why the cylinder & piston need replacing. The first is the impulse line to the carburetor. This saw recently had a new gas tank installed and I am suspicious that the impulse line was not installed properly. Could this allow the saw to run yet make it so lean it fried the piston?
The next area I think could be trouble is the muffler gasket between the 2 parts. The muffler was banged up (any saw not have a dented muffler?) and the gasket is broken where the two parts mate. I am wondering if this could allow rain water to enter the exhaust port if the saw was in the rain and in a position that could allow water to collect in it?
I had to order a complete seal kit in order to get the muffler gasket. Is it wise to reseal the saw even though it doesn't leak. I would not think so but thought I would ask some with more experience.
Thanks
Howard

If your using guages with your vac/pressure tests there's really no need to spray soapy water over the saw looking for a leak, the guages will tell you right away whether there is a leak or not.

Its very easy to overlook the simple things that cause major damage. Example I just heard of not long ago. A 660 that burnt up 4 top ends right in a row. The saw had no air leaks and the rpms were correct, should say appeared correct. The dealer finally sent the saw back to Stihl to see what was going on. It took Stihl about 20 minutes to figure it out. The fuel inlet screen in the carb was partly blocked with crud causing a lean running condition. It was getting enough fuel to run and work but starving just the same for enough fuel to keep the engine cool, merely running lean. The carb was cleaned, no more fried top ends. That whole ordeal could had been prevented if the dealer had a clue on what he was doing, embarrassing to say the least.

In simple terms alot of things can cause scoring, not just air leaks, need to check all the possibilites as you go through that saw so it doesn't burn again...

 

[oscar4883]

No, but if the seals pass a vac/pressure test, I can't see any reason to mess with them.

Good advice. IMO it is way to easy to cause yourself more trouble. Manifolds are way to pricey as well to change on every saw. If they pass tests and show no signs of wear, I'm keepin them.

 

[RiverRat2]

Thall is right,,, Seen it more than once,,, Partially plugged inlet screen,, lets the saw start good and have decent midrange power but its running lean @ WOT,,,,

I would pull the carb and pull the covers,,, the way I check them is to fill the inlet screen cavity/bore with fuel mix till its flush with the top of the carb body then depress the needle valve on the bottom while holding it over your spent/waste fuel jug,,,, that fuel mix should flow through the screen like nobodys business,,, really quickly, if it doesent,,, you better change the screen....
some times you can look @ the screen and it will appear clean,,, but the trash or film/scale is on the backside and you cant see it unless you pic it out and hold it up to the light for a second.... JMHO

Not second guessing you but if it pressure/vac test good,,, All it can be is the carb leaning you out somewhere with scoring on the exh. side only????

 

[Fish]

Yes, a whole lot of things can burn up saw,




first, a redneck with a little screwdriver.

 

[RiverRat2]

Yes, a whole lot of things can burn up saw,




first, a redneck with a little screwdriver.

Yep!!!! aint that amazing how that works,,, Fish is right,,,, I forgot to mention that,,, and when your done putting the carb backtogether put all the settings back to spec,,, and maybe a tad rich wont hurt

I have all the settings if you need them,,,,

 

[challenger]

Wow! thanks for all the replies and good ideas.
I will try to post some pictures of the piston. The cylinder had minor aluminum build up on it but I figured doing the jug and piston was cost effective.
I know the manifold rubber is good-I had it plugged when I did the test on the crank case with a homemade plug made from foam rubber (closed cell) and a fuel line clamp. It took some time but didn't leak. I am also confident the fuel and impulse lines are sound.
I saw one post stating the carb should be set to 1 to 1-1/2 turns? The saw has 1/4 turn for the low side and 3/4 turn for the high side printed on the cover?? I honestly have always adjusted saws by ear so what do I need to do to do a better job of setting the carb?
Good advice on the fuel screen in the carb. I did a kit on another 460 from the same guy today and indeed there was a film on the screen that was transparent but when I went to pick the screen out this gooey snot dragged along with it.
Is it OK to just check the screen without putting a new kit in the carb? I've always put kits in whenever I've opened one. These 2-cycle carbs are so finicky I've done more than several and I'd say that 20% of the time a cleaning and rebuild still leaves me with a carb that will just not be tunable yet put a new carb on and the machine work fine? I know ethenol is the main issue with fuel problems-at least in my area but that should make a carb junk???
I look forward to any carburetor settings that I can get and I appreciate the offer for them and all the advice/ideas.

 

[oscar4883]

Those settings on the cover are probably with the limiter caps intact. Remove them so it can be tuned properly and start around 1 turn on the low and maybe 1 1/4 to 1 1/2 on the high.

 

[THALL10326]

Wow! thanks for all the replies and good ideas.
I will try to post some pictures of the piston. The cylinder had minor aluminum build up on it but I figured doing the jug and piston was cost effective.
I know the manifold rubber is good-I had it plugged when I did the test on the crank case with a homemade plug made from foam rubber (closed cell) and a fuel line clamp. It took some time but didn't leak. I am also confident the fuel and impulse lines are sound.
I saw one post stating the carb should be set to 1 to 1-1/2 turns? The saw has 1/4 turn for the low side and 3/4 turn for the high side printed on the cover?? I honestly have always adjusted saws by ear so what do I need to do to do a better job of setting the carb?
Good advice on the fuel screen in the carb. I did a kit on another 460 from the same guy today and indeed there was a film on the screen that was transparent but when I went to pick the screen out this gooey snot dragged along with it.
Is it OK to just check the screen without putting a new kit in the carb? I've always put kits in whenever I've opened one. These 2-cycle carbs are so finicky I've done more than several and I'd say that 20% of the time a cleaning and rebuild still leaves me with a carb that will just not be tunable yet put a new carb on and the machine work fine? I know ethenol is the main issue with fuel problems-at least in my area but that should make a carb junk???
I look forward to any carburetor settings that I can get and I appreciate the offer for them and all the advice/ideas.

Funny thing about those inlet screens that look clean as can be but aren't clean at all. Those red plastic gas cans that fade to pink and then finally to almost white, those old cans are the main sourse of those screens getting clogged with stuff you can't hardly see. Whats happening with those type of cans is the plastic is actually faking away into the fuel. The particales are so fine most fuel filters won't catch them but the fuel inlet screen will. The really neat part is they are so clear you can't hardly see them building up on the inlet screen. I've seen a few saws that quit running before they fried and found this clear looking paste blocking the inlet screen, that build up was once part of that old faded plastic gas can.

Anytime a saw fries and no air leaks are found the problem usually rests somewhere in the fuel delivery system, either by fault of the system, or as Fish points out, someone made some bad carb adjustments prior to roasting the huzzy. Another clue to look for is the chain if everything else checks out ok. A super dull chain can fry any saw if used long enough..

 

[challenger]

I did try to find out the answer to this question before asking here but didn;t have any luck. I am wondering what others experiences are with rebuilding carburetors successfully. I am talking about carbs that are in good shape from a cosmetic standpoint-not those that look like barnicles are growing in them. It would seem to me that there is little going on inside the carburetor that a good cleaning and rebuild kit shouldn't fix. I know some of the orrifices are super tiny but if cleaned out well???? Perhaps I should post this as a separate thread?
Thanks-Howard

 

[MS260 Fan]

A carb is definitely worth rebuilding. I normally take them apart and give them a good cleaning with carb cleaner and install a new kit. The metering diaphragm can get crusty on an old carb so a $5-8 kit can really do great things. I have a low budget Harbor freight ultrasonic cleaner that I clean carbs in now. Most of my rebuilds before the HF unit worked fine with just a basic clean up and a rebuild kit though.

The setting on the H and L will be the key to not burning a good saw up again. The guys are giving good starting points to set the screws. It took me a while to figure out how to read the H and L settings on the side of the saw. Stihl's are great this way. They give a great starting point when tuning.

Good luck!